Chapter 6: stability and control
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Equations (6.8) and (6.10) give valuable insight into the influence which the wing and tail of a conventional tail-aft airplane exert on its trim diagram. Note that (6.8) reveals that, since  is normally negative or zero and o is normally very small, the incidence angle of the horizontal tail must not be zero if  is to be positive. Note also that it was defined as positive when the horizontal tail is oriented so that it is at a lower angle of attack than the main wing. This makes sense, because when the main wing is producing no lift, the tail , if it > 0, will be at a negative angle of attack. The lift produced by the tail in this situation would be downward, creating a nose-up pitching moment, so that > 0.
Most conventional aircraft are designed and balanced so that their centers of gravity are aft of the aerodynamic centers of their wing/fuselage combination. For this situation, the wing term in (6.10) is positive, and since  < 0 for stability, the wing tends to destabilize the aircraft. The tail term in (6.10) is negative, so the tail must overcome the destabilizing effect of the wing in order to make the airplane stable. Expressions for and  for other aircraft configurations may be developed using the same approach which produced (6.8) and (6.10).
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